In particular, the invention is applicable to horology in order to obtain moulds intended for manufacturing by injection magnetic rotors for the motor assemblies of quartz watches. It is likewise applicable for obtaining moulds for the manufacture of parts by swaging.
In the description to follow, there will be considered by way of example the obtaining of a mould for manufacture of magnetic rotors for quartz watches. However, it is well understood that the invention is not to be considered as limited to this particular mould, but may apply to the obtaining of moulds of any form whatsoever intended for the manufacture of any small dimensioned parts whatsoever.
A magnetic rotor for a quartz watch comprises a magnet in the form of a disc and a part which is injection moulded. This part comprises a housing of generally cylindrical form providing a casing for the magnet, an upper axis including a pinion, a step and a pivot, and a lower axis including a step and a pivot. The magnetic rotor has an axial length of about 3 mm:. and a diameter, for the housing, of about 2 mm. The diameter of the pinion is generally between 0.5 mm. and 1 mm. and the pivots have a diameter on the order of 0.1 mm.
The magnetic rotor is obtained by injection moulding. According to the prior art, the mould may be formed in a metal block by techniques such as spark machining or electro-erosion.
The cutting precision available with these techniques is sufficient to obtain a mould of small dimensions such as the cavity form of a leaf of the pinion of the magnetic rotor. However, it is possible by such techniques to obtain only a simple profile for this cavity form such as for example a profile made up of a series of line segments.
It is known that pinion leaves are generally of complex profiles, for instance of the hypocycloid type in order to permit improved meshing. There thus exists an unsolved problem using known techniques.
The purpose of the invention is to provide a solution to this problem. In particular the invention enables obtaining moulds intended for manufacturing magnetic rotors for quartz watches comprising pinion leaves having a complex profile for a pinion with a diameter less than 1 mm. Generally, the invention aims to obtain moulds for the manufacture of parts of very small dimensions comprising complex profiles and of a size not exceeding some tenths of a millimeter.
The method of the invention consists in obtaining the mould from a powder comprising at least one metal (designated as "metallic powder" in the rest of the description) which is formed by moulding on a matrix having the form of the parts to be manufactured and then sintered.
The method of the invention is totally different from the known method consisting of obtaining the parts from a sintered metallic powder. Effectively, the invention resides in the fact that it obtains a negative of the mould by machining a matrix, then forms a negative of said matrix in order to obtain the mould. This double operation enables the determination of the form of the mould on a part in relief--the matrix--which may be easily worked on while, according to the prior art, the mould is directly obtained as a cavity by hollowing out a part.